The performance of a duplex radio communication system may be impaired and unpredictable unless the mutual coupling between its cosited or collocated antennas is minimized.
For a given separation, it is well known that colinear antennas provide greater interantenna isolation than parallel side-by-side antennas. For example, colinear dipoles separated by 1.5 wavelengths will provide 35 decibels isolation. On the other hand, a separation of 7 wavelengths would be required to obtain 35 decibels isolation with the parallel side-by-side antenna arrangement.
By providing enough spacing, interantenna isolation can, in principle, be reduced to an acceptable level as dictated by system requirements. At VHF, however, where one wavelength may be 10 meters, neither the colinear nor parallel side-by-side antenna arrangement is practical because of the large separation required.